Problem with users spamming place order with onepage checkout

I've just encountered and issue where I either have a customer or a bot spam Place Order on my checkout page with what I assume is fraudulent card numbers. Now no orders have actually been placed due to the card continuously being declined. But I do have 1000+ attempts during the past several days.

Now my question, Is this something that I should be worried about. And is there an easy way to fix this?